1. Field of the Invention
The present invention pertains generally to devices used to measure the distance to an object. More particularly, the present invention pertains to a class of laser radar sensors known as “flash” ladar sensors.
Conventional flash radar sensors use a single laser pulse to measure range to multiple spatial positions. In the present invention, the time delay of light reflected from a succession of laser pulses is measured. The laser pulses are generated at different intervals of an azimuth angle with each pulse lasting for a predetermined interval of time. By processing the signals received during each laser pulse, the range, elevation and angle of an object can be quantified in real time. The present invention uses established charge coupled device (CCD) imaging technology in a novel architecture that improves the speed of pure scanning ladar sensors at a considerable reduction in cost and complexity.
2. Background of the Invention
Ordinarily, the term “Flash Ladar Sensor” refers to an electronic sensor that generates a three-dimensional range profile from the scattered light of a single transmitted laser pulse.
To date, work has been focused on generating a full two-dimensional range image of a region of interest by flood illuminating the region with a single laser pulse and optically capturing the reflected energy on a two-dimensional photosensitive pixel array. These prior art photosensitive pixel arrays require time delay measurement circuitry for each pixel of the array. Considering the already high level of circuit integration that had been used with imaging arrays, the addition of time delay measurement circuitry on a pixel by pixel basis has led to a host of design and manufacturing complexities. Still further, prior art flash ladar designs require very high energy laser pulses in order to flood illuminate a relatively large volume of space.
Instead of building time delay measurement circuitry into each unit cell of the array, the sensor of the present invention uses similar technology and circuitry to that employed with conventional full frame transfer CCD imaging chips.
The pixel array of the full-frame CCD consists of a photosensitive parallel shift register onto which images are optically projected by means of a camera lens. In the full-frame CCD, all of the photodiodes in the two dimensional pixel array collectively act as the image plane and are available for detecting photons during the exposure period. The information contained in each pixel element of the full-frame CCD represents a small portion of the total projected image. For color imaging, each of the pixel elements would contain four photodiodes masked with red, green and blue colored filters.
For monochrome imaging, a single photodiode is used.
After photons composing the image have been collected by the pixel elements and converted into electrical potential, the CCD undergoes readout by shifting rows of image information in a parallel fashion, one row at a time, to a serial shift register. The serial register then sequentially shifts each row of image information to an output amplifier as a serial data stream. The entire process is repeated until all rows of image data are transferred to the output amplifier and off the chip to an analog to digital signal converter integrated circuit. Reconstruction of the image in a digital format yields the final photograph.
The present invention does not pertain to imaging photography; it is concerned with ascertaining the range of an object from the sensor. Unlike the conventional CCD imaging chip that requires all of its pixels to be photosensitive, in the present invention the pixels of CCD pixel array are all masked except for one column of photosensitive pixels. The masked pixels are used as a charge storage memory area.
In the conventional CCD imaging chip, all of the pixels are used to integrate photo-charge for a relatively long integration period (greater than 1 micro second) after which a shutter is closed and the charge from the entire array is read out. By contrast, the present invention does not utilize a shutter and the charge is transferred at a very fast rate (on the order of 10 nanoseconds) while photo-charge is continually integrated on the photosensitive portion of the array. This chip architecture allows the charge storage region of the present invention to retain a time history profile of reflected light.
A high speed imaging device made by DRS Hadland, LTD used for ballistic imaging uses a dimensional imaging array in which only one pixel is unmasked out of a combined pixel group or superpixel (i.e., each superpixel may consist of a 4×4 pixel section of the array). In this type of camera, a fast succession of images is taken by shifting the charge from each photosensitive pixel within the other pixels of its superpixel group so that images of a high speed event are captured. The DRS Hadland, LTD device employs the concept of shifting charge in a masked pixel for producing a two-dimensional image but cannot be used to determine the range of an object.